It is well known that the use of structured packing in distillation columns has many advantages where low pressure drop is important. However, packed column performance is very dependent on creating and maintaining a balance between the downward flow of liquid and upward flow of vapor locally in the packing. The distribution of the liquid and vapor within the packing is influenced by the initial presentation of these fluids to the packing, and the particular characteristics of the packing.
The structured packing typically consists of sheets of corrugated metal, which are bound together in sections, forming a series of criss-crossing open channels. The sections abut each other to form a layer filling the column cross-section. Typically, several layers of sectioned structured packing are stacked on top of one another to an appropriate height to create a packed column. Each layer has a single direction to which the packing sheets and the channels formed by them are parallel. The liquid and vapor spread more easily in the direction of the channels within a given layer and, by following the channels, the fluids from different parts of the column can mix and congregate within the packing. Typically, each layer is angularly orientated by, for example 90.degree., relative to adjacent layers to promote mixing.
In both stationary and moving columns, difficulties arise in maintaining uniform liquid and vapor distribution within structured packing, due to the migration of the fluids across the column cross-section following the channels in the packing. The migration tends to create local imbalances of the liquid and vapor flows, leading to poor distillation performance. One manifestation of this phenomenon is the well-known relationship between mass transfer efficiency and structured packed column diameter: small columns tend to have higher mass transfer rates than larger columns. In theory, the rapid mixing of the entire column contents across the cross-section occurs more readily in a small column, thus mitigating the effects of the local flow imbalances. Non-stationary column installations, such as off-shore applications, pose special problems in maintaining uniform liquid distribution. The periodic tilting of the column tends to force the liquid toward the walls of the column, creating severe flow imbalances between the liquid and vapor in the packing.
An alternative to structured packing is spiral-wound packing, which is composed of a continuous sheet of corrugated packing which is wound on a mandrel, starting from the inside of a column and winding to the outside of the column. A continuous, spiral path for liquid and/or vapor is created between the corrugated sheets as they wind from the inside to the outside of the column. Spiral-wound packed columns have similar difficulties in maintaining flow uniformity, but, because transverse mixing may not occur as readily, may not enjoy the small-column performance enhancement observed in structured packed systems. Furthermore, spiral-wound packed columns have a spiral pathway to the wall of the column which may lead to accumulation of liquid there, especially in a shipboard application. However, a spiral-wound packing may have advantages over a structured packing for some applications because of reduced migration of the liquid and vapor radially in the column.
The prior art attempts to maintain the local balance between liquid and vapor flows within the structured packing by partitioning a layer of packing in a large column into smaller sections. Partitioning forces the liquid flowing within a partitioned section to stay within that section, thus maintaining the balance between liquid and vapor flows.
U.S. Pat. No. 5,486,318 (McKeigue and Krishnamurthy) teaches variations of packing partitioning by the use of physical partitions or by the use of varying packing orientation as a means of partitioning. These variations are depicted in FIG. 1. The physical partition P shown in FIG. 1a is a perforated plate, with perforations large enough that some vapor can pass through, but too small for a significant amount of liquid to flow through. The second variation, shown in FIG. 1b, requires that each section of packing abuts an adjacent section of packing perpendicularly, so that the channels in the packing sheets in Section A, for example, are bordered by a single packing sheet, Sheet C, of abutting Section B. The channels of Sheet C run in a perpendicular plane to those in Section A. Although not pointed out in U.S. Pat. No. 5,486,318, when liquid runs down the channels from several sheets in Section A toward Section B, the liquid will tend to accumulate on a single sheet, Sheet C. The accumulation of liquid from several sheets onto one sheet results in local channelling, and thereby degrades the mass transfer performance of the column.
It is an object of the present invention to provide an arrangement of packing sheets which permits of uniform liquid-vapor contact in a packed column during sway or tilt such as that encountered by a shipboard cryogenic air separation distillation column.